1. Field of the Invention
The present invention relates to the field of photomultipliers and, more particularly, to a coating material and method for reducing the photosensitivity of electron accelerating structures, such as dynodes, to visible and infrared radiation.
2. Description of the Prior Art
Photomultipliers come in a wide variety of forms and generally include a photocathode formed from a material exhibiting photoemissivity, i.e. photons impinging on the material cause electrons to be ejected from the photoemissive material. Various electron accelerating structures, such as focus electrodes and dynodes, are spaced apart from the photocathode. A DC potential is applied to the photocathode, focusing electrodes and dynode with the dynode being made positive with respect to the photocathode so as to attract any electrons ejected from the surface of the photocathode. The electrical potential between the acceleration structures and the photocathode acts to accelerate the photoelectrons ejected from the photocathode toward the dynode. This causes the photoelectrons which impact and collect on the dynode to have a higher energy than when they were initially ejected from the photocathode. The dynode is usually coated with a material, such as beryllium oxide, which exhibits good secondary electron emission: i.e. a primary electron which impacts the surface of the dynode will cause one or more secondary electrons to be emitted from the secondary emissive material coating the dynode.
These secondary electrons may be further accelerated by placing a second dynode a short distance away from the first dynode and applying a DC potential to the second dynode which is positive with respect to the first dynode. The secondary electrons are therefore accelerated toward the second dynode, which is similar in structure to the first dynode. This dynode structure may be repeated for as many stages as desired. The final stage of the photomultiplier is an anode which is made more positive with respect to the last dynode and which serves to collect the secondary electrons being emitted by the last dynode stage.
Since each secondary electron which is ejected from a dynode caused one or more secondary electrons to be ejected from the succeeding dynode, a cascade effect will occur: a single incident photoelectron collected by the first dynode results in several thousands or millions of secondary electrons being collected at the final stage (the anode) of the photomultiplier.
Although the foregoing is descriptive of the general structure of a photomultiplier, it will be appreciated that photomultipliers come in other forms. Thus, the dynodes may be separate structures, or as in the case of a microchannel plate photomultiplier, the dynode is a continuous tubular structure having an electron emissive material coated on its interior. Examples of common types of photomultiplier structures are shown in Photomultiplier Handbook, RCA, 1980, pages 26-35.
Photomultipliers are usually designed for use in specific applications and for detecting photons of a particular range of energies or wavelengths. The particular photocathode materials used will therefore depend upon the particular energies or wavelengths of interest.
One such type of photomultiplier is the so-called "solar blind" photomultiplier which is sensitive to photons in the ultraviolet spectrum. It is desirable that a UV sensitive photomultiplier exhibit little or no response to photons of energies corresponding to wavelengths greater than 350-380 nanometers, i.e. radiation in the visible and infrared spectrum, since this radiation tends to "swamp out" the shorter wavelength UV radiation.
There are many materials which are sensitive to ultraviolet radiation. Such materials include cuprous chloride, sodium chloride, potassium bromide, copper iodide, cesium iodide, cesium telluride, rubidium iodide, rubidium telluride and potassium telluride. The particular material selected will depend upon the portion of the ultraviolet spectrum which is of interest.
For example, cesium telluride (Cs.sub.2 Te) has a quantum efficiency (number of photoelectrons ejected per incident photon) percentage of greater than 2% at wavelengths between approximately 105-300 nanometers. Another common UV sensitive photocathode material is rubidium telluride (Rb.sub.2 Te) which has a quantum efficiency of greater than 2% at wavelengths between approximately 150-290 nanometers.
One problem associated with solar blind photomultipliers is that the alkali metals commonly used in the processing of the photocathode (e.g. cesium, rubidium or potassium) are applied using a gaseous diffusion process. This results in some excess alkali metal being deposited not only on the photocathode but also on the dynodes or other electron accelerating structures, such as the focusing electrodes. This free alkali metal is itself photoemissive and sensitive to photons having energies corresponding to wavelengths in the visible and infrared regions. Since the UV sensitive photocathode materials used in making a solar blind photomultiplier are generally semi-transparent, any visible or infrared radiation impinging on the photomultiplier will strike the dynodes and cause an unwanted response in the output of the photomultiplier.
Due to the nature of the diffusion process used in applying the photosensitive alkali metal components to the photocathode, it is not possible to completely eliminate or remove the free alkali metal from unwanted areas within the photomultiplier.